EDFAs, including multi-stage EDFAs, are known, and are expected to find wide commercial use in optical fiber communication systems. EDFAs can be used as power amplifiers, as pre-amplifiers, or as in-line amplifiers in long (e.g., &gt;100 km) transmission lines. See, for instance, J. M. Delavaux et al., J. Lightwave Technology, Vol. 13(5), p. 703. Although a variety of pump wavelengths have been used, current interest is almost exclusively in EDFAs pumped at about 980 nm or 1480 nm, for signal wavelengths of about 1.55 .mu.m.
Desirable properties of amplifiers are low noise and/or good power conversion efficiency. With regard to EDFAs, it has been recognized that use of high numerical aperture (NA) Er-doped amplifier fiber can improve noise figure, gain and power conversion efficiency. See, for instance, B. Pedersen et al., IEEE Photonics Technology Letters, Vol. 3(12), p. 1085, and T. Kashiwada et al., IEEE Photonics Technology Letters, Vol. 3(3), p. 721. However, recently it was discovered that low NA amplifier fiber can yield high power conversion efficiency when pumping with high power (&gt;500 mW) 980 nm pump radiation. See, for instance, J. C. Livas et al., Proceedings of CLEO, CPD 27-1, May 21-26, 1995, Baltimore, Md.
"NA" herein has the conventional meaning, namely, NA=.sqroot.2n.DELTA.n, where n is the core refractive index, and .DELTA.n=n-n.sub.c1, the un-normalized difference between the core refractive index and n.sub.c1, the refractive index of the inner cladding.
In view of the anticipated wide use of EDFAs in optical fiber communication systems, it would be highly desirable to have available EDFAs that can provide both high power conversion efficiency and low noise figure. This application discloses a multistage EDFA that can provide these advantageous features.